Cut equal pieces of straw from each side of the flat region so that the straw has two lips at the end.
Experiment with blowing hard and softly while biting down with different amounts of pressure until you make the straw sing.
When you blow, a pulse of compressed air flows down the straw. The pulse travels down the straw at Mach 1, the speed of sound, and bounces off the distant open end.
When the sound bounces off the open end, the compressed air changes into a low-pressure expansion. When the expanded air reaches the lips of the straw, they are forced closed - then bounce open to admit more air. The sound bounces back and forth inside the straw and the lips of the straw open and close to create a sound.
If you shorten the straw, the sound takes less time to travel down the straw and back, so the frequency of the sound increases (making the pitch higher). You can demonstrate this by cutting off the end of the straw with scissors. As you snip the end off, the frequency increases.
You can also find one straw that fits into another to make a longer straw with a lower frequency.
If you make holes in the straw, which you can do with a soldering iron, you can make an oboe that you can play by covering the holes with your fingers. An uncovered hole acts as the end of the straw.
If you can find two straws that fit one inside the other and yet slide back and forth, you can make a straw trombone.
You can roll a piece of paper into a cone and tape the cone onto the end of the straw to make a straw oboe with a bell. The bell makes less sound bounce off the end of the straw so that more sound goes out into the air. This makes the sound much louder.
Use poster paper to make a larger bell, and the straw oboe will become very loud.
The straw oboe with a bell acts like an old fashioned ear trumpet in reverse. Instead of collecting sound, it broadcasts sound.